<link rel="stylesheet" href="//fonts.googleapis.com/css?family=Roboto%3A300%2C400%2C500%2C700%7CRoboto+Slab%3A400%2C700">Humans have fragile bones (and it happened recently) - Filthy Monkey Men

Modern humans have a lot of “strengths.” We’re smart, innovative and make awesome tools. Although ironically, strength isn’t one of them. What’s up with that? Chimpanzees, our closest living relatives, are physically stronger than us in almost every way. Even Neanderthals – who are so similar to us we interbred- have significantly stronger bones and muscles. Why are humans the runt of the ape litter? Why are humans weak?

Our bones in particular are especially fragile. Trabecular (or spongy) bone in joints is less dense than standard bone, acting as a shock-absorber. However, humans don’t have much of this type of bone in our joints (sometimes as little as half the trabecular bone of chimps) making our limbs a lot more vulnerable to damage. Why did we evolve such an obvious weakness?

Spongy bone in our joints (from wikipedia)

Popular explanations

Advanced bone tools made by Neanderthals – which they developed before humans invented similar ones

The fact humans are weak is hardly a new discovery. Scientists have been studying this question for years and have developed several popular explanations. Arguably the most popular is that humans don’t need to be strong any more. Our tools and technology have take over for us. We don’t need the muscles to hurl a spear across a field, we can use a bow and arrow instead.

Another popular explanation is that it’s an adaptation to walking upright. All of our body weight is supported by only two legs, compared to the four legs that prop up most other apes. As such, our legs have to work a lot harder to keep us upright; so anything which reduces the weight they have to support would be useful. Even if that lost weight also means we loose some muscle and strength.

Unfortunatley Neanderthals undermine both of these explanations. As I just mentioned, they were a lot stronger than us. Yet their technology wasn’t particularly primitive and they didn’t have much trouble walking either. In short, Homo neanderthalensis seems to have been able to balance being strong with advanced technology and walking upright. So why did humans fail to walk that tightrope? Why are humans weak?

When did human bones become weak?

So, the Neandethals have made many popular arguments for why humans are weak, extinct (haha, get it. Because all the Neanderthals died). However, a recent review of fossil humans’ trabecular bone strength may have uncovered the real reason as to why are human bones are weak. A reason not even the Neanderthals can dispute.

This review examined the limb bones of dozens of modern and fossil humans; along with the limbs of other members of the human family (like Neanderthals and Homo erectus) and our close relatives (like chimps). One surprising trend emerged out of all this data: our weak bones are “new.”

We’re living in a period called the Holocene; which started around 12,000 years ago. Human fossils – and, you know, the people still living today – were found to have relatively little trabecular bone. Yet if you wind the clock back to the period before the Holocene; suddenly we start finding humans with levels of trabecular bone similar to chimps, Neanderthals and the other strong boned species.

why are humans weak
A cross section of different species’ joints; showing how recent humans have the least trabecular bone (the liney structures inside) of the bunch

So why are humans weak?

So what happened in the Holocene that might explain our evolution of weak bones? Well, the Holocene marks the first human experiments with domestication; eventually leading to the invention of farming. The adoption of farming represents the biggest lifestyle change in our family’s history and it seems likely that this might be the source of our weak bones.

Farming is hardly an easy life. Any feudal peasent will tell you that. However, it is a less active lifestyle than being a hunter-gatherer; who often have to travel significant distances to find food. On top of that farming tends to produce a food surplus, allowing people to spend a lot of time doing other things. Things which might not involve that much physical activity.

Nothing in life is free and our bodies are no exception. They require energy to grow and sustain. If evolution can cut a few corners without harming our chance for survival, it will. It’s also worth noting that this is actually a genuine case of evolution in action. The change in trabecular bone levels is greater than the variation within our species. Something new was going on.

tl;dr

Why are humans weak? Well, we physically suck in a lot of ways. One key weakeness is that our joints have less spongy bone than many other species; which normally acts as a shock absorber. Why did humans loose some? With the invention of farming our lifestyle became a lot more sedentary and such physical adaptations were no longer needed.

Reference

Chirchir, H., Kivell, T. L., Ruff, C. B., Hublin, J. J., Carlson, K. J., Zipfel, B., & Richmond, B. G. (2014). Recent origin of low trabecular bone density in modern humans. Proceedings of the National Academy of Sciences, 201411696.

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9 Comments

Jim Birch · 9th January 2015 at 12:05 am

What are the bones of HGs like the Australian aboriginals like?

    Adam Benton · 9th January 2015 at 12:07 am

    I’m not sure about Australians specifically, but they did examine some h/gs and found they had low levels of spongey bone also. This was attributed to the fact they were no longer pure h/g, as they traded with local farmers

welikehumans · 9th January 2015 at 2:06 am

Ummm, chimps and Neanderthals are bad at salsa dancing?

David Atkins · 26th February 2015 at 2:39 pm

I am interested to know the time scale of evolutionary changes. In the case of changes in spongey bone in humans during the Holocene are we talking hundreds of years, thousands of years? Presumably it is a gradual process, so the first farmers circa 10000 years had higher levels of spongey bone than , say, more modern farmers of c. 5000 years or less, and so on. Is there evidence to support this eg can we compare fossil bones of the first farmers in the Fertile Crescent with those of much later farmers in the same region? Ditto for the evolution of farming in other areas eg China, Africa, the Americas.

    Adam Benton · 11th March 2015 at 11:13 am

    That would be a very interesting question to answer; although the paper discussed above doesn’t really go into it. They were more interesting in establishing it is a recent development rather than investigating the finer points of the timeline.

Dave · 11th April 2015 at 10:08 pm

I read that in fact hunter gathering leaves most groups with more rest time than farming. So farming is not more sedentary. Check that for yourself but I think it undermines your hypothesis.

Perhaps farming is simply less brutal and less extreme. Perhaps there are less extreme pressures on the body.

Also studies have shown that hunter gathers have a wider more healthy diet than farmers. It’s thought that at least originally hunter gathering provided more protein per individual. This change in diet may have driven us to become weaker. It could be that farming meant this weakness was not a great disadvantage.

So perhaps your conclusion is correct but it wasn’t due to more leisure time.

    Adam Benton · 22nd June 2015 at 10:10 am

    It is true that some (but not all) hunter-gatherer groups wind up with more free time than farmers. Yet when they do work, it is often more intensive and dangerous. It would certainly be interesting to see how these different factors are connected. Which was the key variable? A changed diet, leisure time, risk level etc.

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